Natural gas has been used as a fuel for internal combustion engines, primarily because it produces less pollution than diesel or gasoline. Previously, natural gas was introduced into the cylinders through the intake manifold, mixed with the intake air and fed into the cylinders at relatively low pressures. The fuel supply system for a natural gas powered engine is relatively simple. The natural gas is held in and supplied from a tank with a working pressure just above the engine inlet pressure, or from compressed natural gas cylinders through regulators that reduce the pressure to the engine inlet pressure.
Compressed natural gas (CNG) is commonly stored at ambient temperatures at pressures up to 3600 psig (24,925 kPa), and may be unsuitable for many conventional trucks and buses due to the limited operating range and the heavy weight of the storage tanks. On the other hand, liquefied natural gas (LNG) is normally stored at temperatures of between about −240° F. and −175° F. (−150° C. and −115° C.) and at pressures of between about 15 and 200 psig (204 and 1477 kPa) in a cryogenic tank, providing an energy density of about four times that of CNG.
However, better efficiency and emissions can be achieved if the natural gas is injected directly into the engine cylinders under high pressure at the end of the compression stroke of the piston. This requires a fuel supply system that can deliver the natural gas at a pressure of about 3000 psig (20,684 kPa) and above. This makes it impossible to deliver the fuel directly from a conventional LNG tank because it is impractical and uneconomical to build an LNG tank with such a high operating pressure. Equally, it is impossible to deliver the natural gas fuel directly from a conventional CNG tank as the pressure in such a tank is lower than the injection pressure as soon as a small amount of fuel has been withdrawn from the CNG tank. Therefore, in both cases, a booster pump is required to boost the pressure from storage pressure to injection pressure.
Booster pumps in the form of high pressure cryogenic pumps are known, but it has proven difficult to adapt these pumps to the size and demand of a vehicle pump. In general, cryogenic pumps should have a positive suction pressure. It has therefore been common practice to place the pump directly in the LNG so that the head of the LNG will supply the desired pressure. The problem with this approach is that it introduces a large heat leak into the LNG storage tank. Some designs place the pump outside the storage tank and have reduced the required suction pressure by using a large first stage suction chamber. The excess LNG which is drawn into such a chamber is returned to the LNG tank and again, additional heat is introduced into the LNG, which is undesirable.
Conventional cryogenic pumps are typically centrifugal pumps, which are placed either in the liquid inside the storage tank, or below the storage tank in a separate chamber with a large suction line leading from the tank, with both the pump and suction line being well insulated. Because a cryogenic liquid is at its boiling temperature when stored, heat is leaked into the suction line and a reduction in pressure will cause vapor to be formed. Thus, if the centrifugal pump is placed outside the tank, vapor is formed and the vapor will cause the pump to cavitate and the flow to stop. Consequently, cryogenic pumps of the centrifugal type require a positive feed pressure to prevent or reduce the tendency to cavitation of the pump. Also, centrifugal pumps cannot easily generate high discharge pressures to properly inject fuel directly into the engine cylinder.
Reciprocating piston pumps have been used for pumping LNG, but such pumps also require a positive feed pressure to reduce efficiency losses that can arise with a relatively high speed piston pump. Such pumps may have a single chamber in which an induction stroke is followed by a discharge stroke, and thus the inlet flow will be stopped half of the time while the piston executes the discharge stroke. U.S. Pat. No. 6,898,940 discloses a dual chamber reciprocating pump that avoids this issue.
The reciprocating piston cryogenic pump of U.S. Pat. No. 6,898,940 is hydraulically actuated. During the compression phase, there is a desire to keep the piston from bottoming out and there is a need to know when to begin the retraction of the piston. One conventional solution is to sense the increase in the hydraulic system pressure as a signal that the end of the compression stroke has been reached and that the retraction stroke should be commenced. However, this solution may still result in bottoming out of the piston at high hydraulic pressures. Another approach uses an integration of the estimated piston velocity to indicate when the end of the compression stroke has been reached. However, this approach is not optimum if there are errors in volumetric efficiency (i.e., leakage) or errors in the hydraulic pressure, gas pressure or consumption measurements. Yet another approach involves placing a position sensor to indicate the end of the compression stroke. However, this design is not robust and will not prevent bottoming out of the piston if there is a failure of the position sensor.
Thus, improved hydraulically activated cryogenic pumps for delivering LNG to internal combustion engines are needed.